A wavelength division multiplex (WDM) system has transmitted an optical signal in which an optical path signal between the WDM system and, for example, an optical transport network (OTN) is subjected to optical wavelength division multiplexing. However, in recent years, a flexible operation of a transmission path has been desired in association with the complicated network configuration such as a ring network and a mesh network.
When a transmission path is changed depending on disconnection of the transmission path on a network and the like, the WDM system also increases or decreases the number of signal wavelengths passing through each optical transmitter on the transmission path. In an optical amplifier inside each optical transmitter, an auto gain control (AGC) system is used for promptly coping with a variation in optical power of an optical signal corresponding to a variation in the number of signal wavelengths. The optical amplifier compares input power of an optical signal in an amplifier input stage with output power of the optical signal in an amplifier output stage, and controls an amplification factor to be a predetermined amplification factor corresponding to a power difference resulting from the comparison.
In the WDM system, a technique will be desired for transmitting optical signals in which an optical path signal and an optical packet signal between the WDM system and a local area network (LAN) such as an Ethernet (registered trademark) network is subjected to optical wavelength division multiplexing. In the WDM system, an optical packet signal is generated at user's use timing, in other words, at random, and an optical packet signal exists in a burst manner in an optical signal. In addition, an optical packet signal has a random signal length and signal interval. In an optical transmitter in the WDM system, input power of an optical amplifier greatly varies depending on whether an optical packet signal exists in an optical signal. Related technologies are described in Japanese Laid-open Patent Publication No. 2007-215147 and Japanese Laid-open Patent Publication No. 2008-166719.
It is assumed that input power is stably input to the AGC-system optical amplifier in an optical transmitter for a predetermined period of time. However, the AGC-system optical amplifier has difficulty in ensuring a stable amplification factor since input power greatly varies depending on whether an optical packet signal exists in an optical signal and a power difference thereof greatly varies. When an optical amplifier obtains a monitoring result indicating that an optical packet signal exists in an optical signal in an amplifier input stage and no optical packet signal exists in the optical signal in an amplifier output stage at an identical monitoring timing, a power difference thereof greatly varies and the optical amplifier has difficulty in ensuring a stable amplification factor.